Currently widely used display devices in which liquid crystal compounds are incorporated are usually driven by TN (twisted nematic) mode.
When driving by TN mode is adopted, however, the positions of liquid crystal compound molecules in an element of the device must be altered in order to change a displayed image. As a result, there are involved such problems that the driving time of the device is prolonged, and the voltage required for altering positions of the liquid crystal compound molecules, i.e., power consumption, becomes large.
Switching elements incorporating ferroelectric liquid crystal compounds, different from those in which TN mode or STN mode is utilized, can function only by altering the molecular orientation direction of the liquid crystal compounds, and hence the switching time is prominently shortened. Further, the value Ps.times.E obtained from a spontaneous polarization (Ps) of the ferroelectric liquid crystal compound and an intensity of the electric field (E) applied is an effective energy output for altering the molecular orientation direction of the liquid crystal compound, and accordingly the power consumption is also significantly diminished. Such ferroelectric liquid crystal compounds as mentioned above have two stable states, namely, bistability, in accordance with the direction of the applied electric field, and therefore show very excellent switching threshold value characteristics. Accordingly, the ferroelectric liquid crystal compounds are particularly suitable for display devices for animations.
When these ferroelectric liquid crystal compounds are used in optical switching elements, etc., they are required to have various characteristics such as an operating temperature in the vicinity of or not higher than room temperature, a wide operating temperature range, a high switching speed (quick), and a switching threshold value voltage in an appropriate range. Of these characteristics, the operating temperature range is a particularly important property when the ferroelectric liquid crystal compounds are put into practical use.
So far as ferroelectric liquid crystal compounds known hitherto are concerned, however, they have drawbacks such as a generally narrow operating temperature range, and an operating temperature range in a high temperature region not including room temperature even when their operating temperature range is wide, as described, for example, in R. B. Meyer et. al., "J. de Phys." Vol. 36 L, p. 69 (1975) and a paper reported by Masaaki Taguchi and Takamasa Harada, "Proceedings of Eleventh Conference on Liquid Crystal" p. 168 (1985). Thus, there are no available ferroelectric liquid crystal compound that is satisfactory from the standpoint of practical use.